There is a need for highly accurate and responsive environmental sensor systems that are low in weight, non-obtrusive, economical to manufacture and able to withstand harsh environments. Typical applications in need of such sensor systems are smart skins for aircraft and smart supporting structures for space stations. Such structures must be able to provide continuous indications of their physical state in real time, be extremely hardy and long lasting, be able to detect small changes over a large area of structure and have a large dynamic range, without interfering with other functions within the structure.
Optical pass band filters such as intra-core fiber gratings, Fabry-Perot etalon based filters, and acousto-optic filters have been proposed for fiber optic sensors. Fiber Fabry-Perot etalon based filters are available from Micron Optics and tunable acousto-optic filters are available from New Focus, Inc. under license from Bell Communication Research.
The fiber Fabry-Perot etalon based filters consist of two mirrored surfaces that can be controllably separated by piezoelectric drivers. Light is transmitted most fully when there is an integral number of wavelengths between the two mirrors and most strongly reflected when there is an integral number of waves plus half a wave of separation. Fiber gratings are constructed by doping an optical fiber with material such as germania. The side of the fiber is then exposed to an interference pattern of radiation to produce multiple variations in the refractive index within the fiber that are very closely and accurately spaced. By adjusting the fringe spacing of the interference pattern, the periodic index of refraction produced can be varied as desired.
The reflecting center wavelength of the spectral envelope of a fiber grating changes linearly with temperature and strain. For a temperature change .DELTA.T and a strain .epsilon., the fractional Bragg wavelength shift is given by: ##EQU1## where .varies. is the thermal expansion coefficient of the fiber, .xi. represents the thermal optic coefficient or dn/dT of the doped silica core material and p.sub.e is the photo elastic constant. For temperature, the index change is the predominant effect, being about fifteen times greater than the expansion coefficient. As reported by W. W. Morey, Distributed Fiber Grating Sensors, Proceedings Of The Seventh Optical Fiber Sensors Conference, p. 285-288, Sydney, Australia, December, 1990, temperature responses of fiber gratings vary with fiber type, but they have found to have been linear up to 500.degree. C. Typical temperature responses are 0.0043 nm./.degree.C. at 833 nm. for Andrew PM fiber and 0.0074 nm./.degree.C. for Corning FlexCore Fiber at 824 nm. When the fiber grating is strained, the Bragg wavelength changes to photoelastically induce a change in the refractive index. For silica, the photoelastic constant is 0.22. Bragg wavelength changes under tension have been measured to 45 kpsi stress, giving a 2.3 nm. shift for a slope of 5.2.times.10.sup.-4 nm. per microstrain at 820 nm. The fiber gratings can be used both as multiplexed distributed grating sensors, and time and frequency division multiplexed sensors, as described in W. W. Morey. However, there has been a need to integrate such fiber gratings or other optical band pass filters into practical and economical sensor systems that can be manufactured using available components.